Unmanned Systems Technology 033 l SubSeaSail Gen6 USSV l Servo actuators focus l UAVs insight l Farnborough 2020 update l Transforma XDBOT l Strange Development REVolution l Radio telemetry focus

67 Development has produced a new engine entitled the REVolution, which is designed primarily for UAVs but can also be customised for marine, road and other vehicle applications. The REVolution is a supercharged two-stroke inline two-cylinder measuring roughly 400 x 350 x 300 mm, with a maximum output of 220 hp (164 kW) – which it achieves at 6800 rpm – and a BSFC of 221 g/kWh when performing at these levels. It had been trialled at up to 7000 rpm at the time of writing, with 8000 rpm as the expected redline. It runs on gasoline, has a BMEP of 25 bar, and weighs 49.89 kg (110 lb) including the oil pump, supercharger and a front-end accessory transmission system that drives both of them. The standard-issue electric fuel pump and dry oil sump are external to the main design (although a pan for collecting oil via gravity is bolted under the crankcase). Thermal management is provided largely by an internal water-cooling system, and integral oiling channels provide further cooling, along with lubrication. The REVolution’s origins Between 2008 and 2010, Krzeminski and his team were performing some work on a four-cylinder, four-stroke, carburetted engine to be used on a UAV – which left Krzeminski somewhat nonplussed. “What surprised me was how heavy it was – and still is – relative to its actual power output,” he says. “There are ways to improve it: it’s a well-known model, and engineers slap turbochargers and fuel injection systems onto it all the time, but the base architecture is heavy and outdated. “We determined that a much higher power-to-weight ratio would be necessary to draw users away from that comfortable, suboptimal-but-reliable type of engine to a newer, more innovative one.” That motivated Strange Development to start considering what such an engine would look like. Soon after, the team started working on software designs and CAD simulations for a poppet valve-controlled two-stroke. This design was inspired by Detroit Diesel’s uniflow-scavenged configuration (referred to in the Apple Tree Innovation dossier, UST 28, October/November 2019) but in late 2012 the team decided against this approach. “With poppet valves, you’d get all the mechanical complexity, cost and weight of a four-stroke valvetrain, and Detroit Diesel’s engine was designed to run at maybe half the revs we were going to need,” Krzeminski says. “The valvetrain dynamics would be great on a diesel, but not for the gasoline engine speeds we were looking to tackle. It also just wasn’t going to get us the power-to-weight ratio we wanted, so we shelved that design temporarily and focused on our engineering services for a while.” In 2014, the company was asked to provide r&d for a two-stroke that used turbocharging to compensate for altitude effects. This spurred an in-house discussion about forced induction, with a focus on how best to control the flow of gases through the two-stroke combustion chamber to boost power output. “That four-stroke engine manufacturer I mentioned earlier, for example, will always put the turbo after the combustion chamber, because you’d want a measure of the wave dynamics to push the charge air back into the chamber,” Krzeminski says. “And then the client asked us about supercharging their design. So we built a test engine, put it on the dyno, and witnessed it actually losing power, Strange Development REVolution | Dossier The client asked us about supercharging their design, so we built a test engine, put it on the dyno and it actually lost power Unmanned Systems Technology | August/September 2020 The REVolution’s BSFC depicted as a function of speed and BMEP